Contextual learning and cue association in fear conditioning in mice: a strain comparison and a lesion study

Contextual memory engrams, and the neuromodulatory influence of the locus coeruleus

Here, we review the basis of contextual memory at a conceptual and cellular level. We begin with an overview of the philosophical foundations of traversing space, followed by theories covering the material bases of contextual representations in the hippocampus (engrams), exploring functional characteristics of the cells and subfields within. Next, we explore various methodological approaches for investigating contextual memory engrams, emphasizing plasticity mechanisms. This leads us to discuss the role of neuromodulatory inputs in governing these dynamic changes. We then outline a recent hypothesis involving noradrenergic and dopaminergic projections from the locus coeruleus (LC) to different subregions of the hippocampus, in sculpting contextual representations, giving a brief description of the neuroanatomical and physiological properties of the LC. Finally, we examine how activity in the LC influences contextual memory processes through synaptic plasticity mechanisms to alter hippocampal engrams. Overall, we find that phasic activation of the LC plays an important role in promoting new learning and altering mnemonic processes at the behavioral and cellular level through the neuromodulatory influence of NE/DA in the hippocampus. These findings may provide insight into mechanisms of hippocampal remapping and memory updating, memory processes that are potentially dysregulated in certain psychiatric and neurodegenerative disorders.

Comparison of three common inbred mouse strains reveals substantial differences in hippocampal GABAergic interneuron populations and in vitro network oscillations

A major challenge in neuroscience is to pinpoint neurobiological correlates of specific cognitive and neuropsychiatric traits. At the mesoscopic level, promising candidates for establishing such connections are brain oscillations that can be robustly recorded as local field potentials with varying frequencies in the hippocampus in vivo and in vitro. Inbred mouse strains show natural variation in hippocampal synaptic plasticity (e.g. long-term potentiation), a cellular correlate of learning and memory. However, their diversity in expression of different types of hippocampal network oscillations has not been fully explored. Here, we investigated hippocampal network oscillations in three widely used inbred mouse strains: C57BL/6J (B6J), C57BL/6NCrl (B6N) and 129S2/SvPasCrl (129) with the aim to identify common oscillatory characteristics in inbred mouse strains that show aberrant emotional/cognitive behaviour (B6N and 129) and compare them to "control" B6J strain. First, we detected higher gamma oscillation power in the hippocampal CA3 of both B6N and 129 strains. Second, higher incidence of hippocampal sharp wave-ripple (SPW-R) transients was evident in these strains. Third, we observed prominent differences in the densities of distinct interneuron types and CA3 associative network activity, which are indispensable for sustainment of mesoscopic network oscillations. Together, these results add further evidence to profound physiological differences among inbred mouse strains commonly used in neuroscience research.

Mbnl2 loss alters novel context processing and impairs object recognition memory

Patients with myotonic dystrophy type I (DM1) demonstrate visuospatial dysfunction and impaired performance in tasks requiring recognition or memory of figures and objects. In DM1, CUG expansion RNAs inactivate the muscleblind-like (MBNL) proteins. We show that constitutive Mbnl2 inactivation in Mbnl2^ΔE2/ΔE2 mice selectively impairs object recognition memory in the novel object recognition test. When exploring the context of a novel arena in which the objects are later encountered, the Mbnl2^ΔE2/ΔE2 dorsal hippocampus responds with a lack of enrichment for learning and memory-related pathways, mounting instead transcriptome alterations predicted to impair growth and neuron viability. In Mbnl2^ΔE2/ΔE2 mice, saturation effects may prevent deployment of a functionally relevant transcriptome response during novel context exploration. Post-novel context exploration alterations in genes implicated in tauopathy and dementia are observed in the Mbnl2^ΔE2/ΔE2 dorsal hippocampus. Thus, MBNL2 inactivation in patients with DM1 may alter novel context processing in the dorsal hippocampus and impair object recognition memory.

Functional dissociation along the rostrocaudal axis of Japanese quail hippocampus

The mammalian hippocampus (Hp) can be functionally segregated along its septotemporal axis, with involvement of dorsal hippocampus (dHp) in spatial memory and ventral hippocampus (vHp) in stress responses and emotional behaviour. In the present study, we investigate comparable functional segregation in proposed homologues within the avian brain. Using Japanese quail (Coturnix Japonica), we report that bilateral lesions of the rostral hippocampus (rHp) produce robust deficits in a spatial Y-maze discrimination (YMD) test while sparing performance during contextual fear conditioning (CFC), comparable to results from lesions to homologous regions in mammals. In contrast, caudal hippocampus (cHp) lesions failed to produce deficits in either CFC or YMD, suggesting that, unlike mammals, both cHp and rHp of birds can support emotional behavior. These observations demonstrate functional segregation along the rostrocaudal axis of the avian Hp that is comparable in part to distinctions seen along the mammalian hippocampal septotemporal axis.

Lateral Fluid Percussion Injury Causes Sex-Specific Deficits in Anterograde but Not Retrograde Memory

Cognitive impairment is a common symptom after traumatic brain injury (TBI). Memory, in particular, is often disrupted during chronic post-injury recovery. To understand the sex-specific effects of brain injury on retrograde and anterograde memory, we examined paired associate learning (PAL), spatial learning and memory, and fear memory after lateral fluid percussion TBI. We hypothesized that male and female mice would display unique memory deficits after TBI. PAL task acquisition was initiated via touchscreen operant conditioning 22 weeks before sham injury or TBI. Post-injury PAL testing occurred 7 weeks post-injury. Barnes maze and fear conditioning were completed at 14- and 15-weeks post-injury, respectively. Contrary to our expectations, behavioral outcomes were not primarily influenced by TBI. Instead, sex-specific differences were observed in all tasks which exposed task-specific trends in male TBI mice. Male mice took longer to complete the PAL task, but this was not affected by TBI and did not compromise the ability to make a correct choice. Latency to reach the goal box decreased across testing days in Barnes maze, but male TBI mice lagged in improvement compared to all other groups. Use of two learning indices revealed that male TBI mice were deficient in transferring information from 1 day to the next. Finally, acquisition and contextual retention of fear memory were similar between all groups. Cued retention of the tone-shock pairing was influenced by both injury and sex. Male sham mice displayed the strongest cued retention of fear memory, evidenced by increased freezing behavior across the test trial. In contrast, male TBI mice displayed reduced freezing behavior with repetitive tone exposure. An inverse relationship in freezing behavior to tone exposure was detected between female sham and TBI mice, although the difference was not as striking. Together, these studies show that retrograde memory is intact after lateral TBI. However, male mice are more vulnerable to post-injury anterograde memory deficits. These behaviors were not associated with gross pathological change near the site injury or in subcortical brain regions associated with memory formation. Future studies that incorporate pre- and post-injury behavioral analysis will be integral in defining sex-specific memory impairment after TBI.

Dysregulation of Npas4 and Inhba expression and an altered excitation-inhibition balance are associated with cognitive deficits in DBA/2 mice

Differences in the learning associated transcriptional profiles between mouse strains with distinct learning abilities could provide insight into the molecular basis of learning and memory. The inbred mouse strain DBA/2 shows deficits in hippocampus-dependent memory, yet the transcriptional responses to learning and the underlying mechanisms of the impairments are unknown. Comparing DBA/2J mice with the reference standard C57BL/6N mouse strain we verify an enhanced susceptibility to kainic acid induced seizures, confirm impairments in hippocampus-dependent spatial memory tasks and uncover additional behavioral abnormalities including deficits in hippocampus-independent learning. Surprisingly, we found no broad dysfunction of the DBA/2J strain in immediate early gene (IEG) activation but instead report brain region-specific and gene-specific alterations. The learning-associated IEGs Arc, c-Fos, and Nr4a1 showed no DBA/2J deficits in basal or synaptic activity induced gene expression in hippocampal or cortical primary neuronal cultures or in the CA1, CA3, or retrosplenial cortex following spatial object recognition (SOR) training in vivo. However, the parietal cortex showed reduced and the dentate gyrus showed enhanced SOR-evoked induction of most IEGs. All DBA/2J hippocampal regions exhibited elevated basal expression of inhibin β A (Inhba) and a learning-associated superinduction of the transcription factor neuronal Per-Arnt-Sim domain protein 4 (Npas4) known to regulate the synaptic excitation-inhibition balance. In line with this, CA1 pyramidal neurons of DBA/2J mice showed fewer inhibitory and more excitatory miniature postsynaptic currents but no alteration in most other electrophysiological properties or gross dendritic morphology. The dysregulation of Npas4 and Inhba expression and synaptic connectivity may underlie the cognitive deficits and increased susceptibility to seizures of DBA/2J mice.

Interactions Between Experience, Genotype and Sex in the Development of Individual Coping Strategies

Coping strategies, the first line of defense against adversities, develop through experience. There is consistent evidence that both genotype and sex contribute to the development of dysfunctional coping, leading to maladaptive outcomes of adverse experiences or to adaptive coping that fosters rapid recovery even from severe stress. However, how these factors interact to influence the development of individual coping strategies is just starting to be investigated. In the following review, we will consider evidence that experience, sex, and genotype influence the brain circuits and neurobiological processes involved in coping with adversities and discuss recent results pointing to the specific effects of the interaction between early experiences, genotype, and stress in the development of functional and dysfunctional coping styles.

Revisiting systems consolidation and the concept of consolidation

For more than 50 years, knowledge of memory processes has been based on the consolidation hypothesis, which postulates that new memories require time to become stabilized. Two forms of the consolidation model exist. The Cellular Consolidation concept is based upon retrograde amnesia induced by amnesic treatments, the severity of which decreases as the learning to treatment increases over minutes or hours. In contrast, The Systems Consolidation model is based on post-training hippocampal lesions, which produce more severe retrograde amnesia when induced after days than after weeks. Except for the temporal parameters, Cellular and Systems Consolidation show many similarities. Here we propose that Systems consolidation, much as Cellular Consolidation (see Gisquet- Verrier and Riccio, 2018), can be explained in terms of a form of state-dependency. Accordingly, lesions of the hippocampus induce a change in the internal state of the animal, which disrupts retrieval processes. But the effect of contextual change is known to decrease with the length of the retention intervals, consistent with time-dependent retrograde amnesia. We provide evidence supporting this new view.

Elemental and Configural Associative Learning in Spatial Tasks: Could Zebrafish be Used to Advance Our Knowledge?

Spatial learning and memory have been studied for several decades. Analyses of these processes pose fundamental scientific questions but are also relevant from a biomedical perspective. The cellular, synaptic and molecular mechanisms underlying spatial learning have been intensively investigated, yet the behavioral mechanisms/strategies in a spatial task still pose unanswered questions. Spatial learning relies upon configural information about cues in the environment. However, each of these cues can also independently form part of an elemental association with the specific spatial position, and thus spatial tasks may be solved using elemental (single CS and US association) learning. Here, we first briefly review what we know about configural learning from studies with rodents. Subsequently, we discuss the pros and cons of employing a relatively novel laboratory organism, the zebrafish in such studies, providing some examples of methods with which both elemental and configural learning may be explored with this species. Last, we speculate about future research directions focusing on how zebrafish may advance our knowledge. We argue that zebrafish strikes a reasonable compromise between system complexity and practical simplicity and that adding this species to the studies with laboratory rodents will allow us to gain a better understanding of both the evolution of and the mechanisms underlying spatial learning. We conclude that zebrafish research will enhance the translational relevance of our findings.

Cue-dependent safety and fear learning in a discriminative auditory fear conditioning paradigm in the mouse

Discrimination between sensory stimuli associated with safety and threat is crucial for behavioral decisions. Discriminative conditioning paradigms with two acoustic conditioned stimuli (one paired with shock [CS+], the other unpaired with shock [CS−]) have been widely used as an experimental model for fear learning. However, no attention has been paid to the effect of the CS− on safety in the paradigms, because the CS− served as a neutral cue or elevated the freezing level due to fear generalization although less effectively than the CS+. By using a noise and a tone as two acoustic CSs in a discriminative auditory fear conditioning (AFC) paradigm, here we demonstrate that mice learn safety for the CS− while showing fear for the CS+ with opposing emotional behaviors. We found that after learning mice exhibited a significant suppression of context-dependent freezing during the CS−, but not during the CS+, indicating learned safety without fear generalization for the CS−. In contrast, the mice showed an enhanced level of freezing during the CS+ even in a novel spatial context, indicating cued fear for the CS+. Moreover, the CS+ also induced rapid defensive behaviors, whereas the CS− disinhibited normal exploratory behaviors. On the other hand, mice showed no significant suppression of contextual fear during the CS− in a paradigm with a pair of tone CSs at different frequencies, although they clearly discriminated the two tones. These results suggest our AFC paradigm with the noise and tone CSs as a useful experimental model for cue-dependent discriminative learning of safety and threat.

Current understanding of fear learning and memory in humans and animal models and the value of a linguistic approach for analyzing fear learning and memory in humans

Fear is an emotion that serves as a driving factor in how organisms move through the world. In this review, we discuss the current understandings of the subjective experience of fear and the related biological processes involved in fear learning and memory. We first provide an overview of fear learning and memory in humans and animal models, encompassing the neurocircuitry and molecular mechanisms, the influence of genetic and environmental factors, and how fear learning paradigms have contributed to treatments for fear-related disorders, such as posttraumatic stress disorder. Current treatments as well as novel strategies, such as targeting the perisynaptic environment and use of virtual reality, are addressed. We review research on the subjective experience of fear and the role of autobiographical memory in fear-related disorders. We also discuss the gaps in our understanding of fear learning and memory, and the degree of consensus in the field. Lastly, the development of linguistic tools for assessments and treatment of fear learning and memory disorders is discussed.

Sound check, stage design and screen plot - how to increase the comparability of fear conditioning and fear extinction experiments

In the recent decade, fear conditioning has evolved as a standard procedure for testing cognitive abilities such as memory acquisition, consolidation, recall, reconsolidation, and extinction, preferentially in genetically modified mice. The reasons for the popularity of this powerful approach are its ease to perform, the short duration of training and testing, and its well-described neural basis. So why to bother about flaws in standardization of test procedures and analytical routines? Simplicity does not preclude the existence of fallacies. A short survey of the literature revealed an indifferent use of acoustic stimuli in terms of quality (i.e., white noise vs. sine wave), duration, and intensity. The same applies to the shock procedures. In the present article, I will provide evidence for the importance of qualitative and quantitative parameters of conditioned and unconditioned stimuli for the experimental outcome. Moreover, I will challenge frequently applied interpretations of short-term vs. long-term extinction and spontaneous recovery. On the basis of these concerns, I suggest a guideline for standardization of fear conditioning experiments in mice to improve the comparability of the experimental data.

Contextual fear conditioning in zebrafish

Zebrafish are a genetically tractable vertebrate that hold considerable promise for elucidating the molecular basis of behavior. Although numerous recent advances have been made in the ability to precisely manipulate the zebrafish genome, much less is known about many aspects of learning and memory in adult fish. Here, we describe the development of a contextual fear conditioning paradigm using an electric shock as the aversive stimulus. We find that contextual fear conditioning is modulated by shock intensity, prevented by an established amnestic agent (MK-801), lasts at least 14 d, and exhibits extinction. Furthermore, fish of various background strains (AB, Tu, and TL) are able to acquire fear conditioning, but differ in fear extinction rates. Taken together, we find that contextual fear conditioning in zebrafish shares many similarities with the widely used contextual fear conditioning paradigm in rodents. Combined with the amenability of genetic manipulation in zebrafish, we anticipate that our paradigm will prove to be a useful complementary system in which to examine the molecular basis of vertebrate learning and memory.

Paradoxical effects of 137Cs irradiation on pharmacological stimulation of reactive oxygen species in hippocampal slices from apoE2 and apoE4 mice

In humans, apoE, which plays a role in repair, is expressed in three isoforms: E2, E3, and E4. E4 is a risk factor for age-related cognitive decline (ACD) and Alzheimer's disease (AD), particularly in women. In contrast, E2 is a protective factor for ACD and AD. E2 and E4 might also differ in their response to cranial ¹³⁷Cs irradiation, a form of radiation typically used in a clinical setting for the treatment of cancer. This might be mediated by reactive oxygen species (ROS) in an-apoE isoform-dependent fashion. E2 and E4 female mice received sham-irradiation or cranial irradiation at 8 weeks of age and a standard mouse chow or a diet supplemented with the antioxidant alpha-lipoic acid (ALA) starting at 6 weeks of age. Behavioral and cognitive performance of the mice were assessed 12 weeks later. Subsequently, the generation of ROS in hippocampal slices was analyzed. Compared to sham-irradiated E4 mice, irradiated E4 mice showed enhanced spatial memory in the water maze. This was associated with increased hippocampal PMA-induction of ROS. Similar effects were not seen in E2 mice. Irradiation increased endogenous hippocampal ROS levels in E2 mice while decreasing those in E4 mice. NADPH activity and MnSOD levels were higher in sham-irradiated E2 than E4 mice. Irradiation increased NADPH activity and MnSOD levels in hemi brains of E4 mice but not in those of E2 mice. ALA did not affect behavioral and cognitive performance or hippocampal formation of ROS in either genotype. Thus, apoE isoforms modulate the radiation response.

Zebrafish and relational memory: Could a simple fish be useful for the analysis of biological mechanisms of complex vertebrate learning?

Analysis of the zebrafish allows one to combine two distinct scientific approaches, comparative ethology and neurobehavioral genetics. Furthermore, this species arguably represents an optimal compromise between system complexity and practical simplicity. This mini-review focuses on a complex form of learning, relational learning and memory, in zebrafish. It argues that zebrafish are capable of this type of learning, and it attempts to show how this species may be useful in the analysis of the mechanisms and the evolution of this complex brain function. The review is not intended to be comprehensive. It is a short opinion piece that reflects the author's own biases, and it draws some of its examples from the work coming from his own laboratory. Nevertheless, it is written in the hope that it will persuade those who have not utilized zebrafish and who may be interested in opening their research horizon to this relatively novel but powerful vertebrate research tool.

Behavioral mechanisms of context fear generalization in mice

There is growing interest in generalization of learned contextual fear, driven in part by the hypothesis that mood and anxiety disorders stem from impaired hippocampal mechanisms of fear generalization and discrimination. However, there has been relatively little investigation of the behavioral and procedural mechanisms that might control generalization of contextual fear. We assessed the relative contribution of different contextual features to context fear generalization and characterized how two common conditioning protocols—foreground (uncued) and background (cued) contextual fear conditioning—affected context fear generalization. In one experiment, mice were fear conditioned in context A, and then tested for contextual fear both in A and in an alternate context created by changing a subset of A's elements. The results suggest that floor configuration and odor are more salient features than chamber shape. A second experiment compared context fear generalization in background and foreground context conditioning. Although foreground conditioning produced more context fear than background conditioning, the two procedures produced equal amounts of generalized fear. Finally, results indicated that the order of context tests (original first versus alternate first) significantly modulates context fear generalization, perhaps because the original and alternate contexts are differentially sensitive to extinction. Overall, results demonstrate that context fear generalization is sensitive to procedural variations and likely reflects the operation of multiple interacting psychological and neural mechanisms.

Impact of Life History on Fear Memory and Extinction

Behavioral profiles are strongly shaped by an individual's whole life experience. The accumulation of negative experiences over lifetime is thought to promote anxiety-like behavior in adulthood (“allostatic load hypothesis”). In contrast, the “mismatch hypothesis” of psychiatric disease suggests that high levels of anxiety-like behavior are the result of a discrepancy between early and late environment. The aim of the present study was to investigate how different life histories shape the expression of anxiety-like behavior and modulate fear memory. In addition, we aimed to clarify which of the two hypotheses can better explain the modulation of anxiety and fear. For this purpose, male mice grew up under either adverse or beneficial conditions during early phase of life. In adulthood they were further subdivided in groups that either matched or mismatched the condition experienced before, resulting in four different life histories. The main results were: (i) Early life benefit followed by late life adversity caused decreased levels of anxiety-like behavior. (ii) Accumulation of adversity throughout life history led to impaired fear extinction learning. Late life adversity as compared to late life benefit mainly affected extinction training, while early life adversity as compared to early life benefit interfered with extinction recall. Concerning anxiety-like behavior, the results do neither support the allostatic load nor the mismatch hypothesis, but rather indicate an anxiolytic effect of a mismatched early beneficial and later adverse life history. In contrast, fear memory was strongly affected by the accumulation of adverse experiences over the lifetime, therefore supporting allostatic load hypothesis. In summary, this study highlights that anxiety-like behavior and fear memory are differently affected by specific combinations of adverse or beneficial events experienced throughout life.

No effect of glucose administration in a novel contextual fear generalization protocol in rats

The excessive transfer of fear acquired for one particular context to similar situations has been implicated in the development and maintenance of anxiety disorders, such as post-traumatic stress disorder. Recent evidence suggests that glucose ingestion improves the retention of context conditioning. It has been speculated that glucose might exert that effect by ameliorating hippocampal functioning, and may hold promise as a therapeutic add-on in traumatized patients because improved retention of contextual fear could help to restrict its generalization. However, direct data regarding the effect of glucose on contextual generalization are lacking. Here, we introduce a new behavioral protocol to study such contextual fear generalization in rats. In adult Wistar rats, our procedure yields a gradient of generalization, with progressively less freezing when going from the original training context, over a perceptually similar generalization context, to a markedly dissimilar context. Moreover, we find a flattening of the gradient when the training-test interval is prolonged with 1 week. We next examine the effect of systemic glucose administration on contextual generalization with this novel procedure. Our data do not sustain generalization-reducing effects of glucose and question its applicability in traumatic situations. In summary, we have developed a replicable contextual generalization procedure for rats and demonstrate how it is a valuable tool to examine the neurobiological correlates and test pharmacological interventions pertaining to an important mechanism in the etiology of pathological anxiety.

A single low dose of valproic acid in late prenatal life alters postnatal behavior and glutamic acid decarboxylase levels in the mouse

RATIONALE

Rodents exposed to valproic acid (VPA) in prenatal life exhibit post-natal characteristics analogous to autism spectrum disorder (ASD). Many previous studies used relatively high doses of VPA during early pregnancy, potentially confounding interpretation because the offspring are the 'survivors' of a toxic insult. Low dose or late gestation exposure has not been widely studied.

OBJECTIVES

We examined the behavioral sequelae of late gestation exposure to low dose VPA in the mouse. We also examined postnatal levels of glutamic acid decarboxylase (GAD65 and GAD67) as markers for GABA neurons, because GABA pathology and subsequent excitatory/inhibitory imbalance is strongly implicated in ASD.

METHODS

Pregnant C57BL/6N mice received a single subcutaneous injection of 100 or 200mg/kg on gestation day 17. The control group received a saline injection on the same day. The offspring were tested in a battery of behavioral tests in adolescence and adulthood. Six brain regions were harvested and GAD65 and GAD67 were measured by western blotting.

RESULTS

Compared to saline-exposed controls, adult mice exposed to prenatal VPA had impaired novel object exploration and fear conditioning anomalies. GAD67 was decreased in midbrain, olfactory bulb, prefrontal cortex and increased in cerebellum, hippocampus and striatum; GAD65 was decreased in all 6 regions.

CONCLUSIONS

Our results suggest that a low dose of VPA in late pregnancy has persistent effects on brain development, and in particular the GABA system, which may be relevant to ASD. Further attention to the impact of gestation time and dose of exposure in VPA-induced ASD models is encouraged.

The Role of Ephs and Ephrins in Memory Formation

The ability to efficiently store memories in the brain is a fundamental process and its impairment is associated with multiple human mental disorders. Evidence indicates that long-term memory formation involves alterations of synaptic efficacy produced by modifications in neural transmission and morphology. The Eph receptors and their cognate ephrin ligands have been shown to be involved in these key neuronal processes by regulating events such as presynaptic transmitter release, postsynaptic glutamate receptor conductance and trafficking, synaptic glutamate reuptake, and dendritic spine morphogenesis. Recent findings show that Ephs and ephrins are needed for memory formation in different organisms. These proteins participate in the formation of various types of memories that are subserved by different neurons and brain regions. Ephs and ephrins are involved in brain disorders and diseases with memory impairment symptoms, including Alzheimer's disease and anxiety. Drugs that agonize or antagonize Ephs/ephrins signaling have been developed and could serve as therapeutic agents to treat such diseases. Ephs and ephrins may therefore induce cellular alterations mandatory for memory formation and serve as a target for pharmacological intervention for treatment of memory-related brain diseases.

Chronic up-regulation of the SHH pathway normalizes some developmental effects of trisomy in Ts65Dn mice

Down Syndrome (DS) is a highly complex developmental genetic disorder caused by trisomy for human chromosome 21 (Hsa21). All individuals with DS exhibit some degree of brain structural changes and cognitive impairment; mouse models such as Ts65Dn have been instrumental in understanding the underlying mechanisms. Several phenotypes of DS might arise from a reduced response of trisomic cells to the Sonic Hedgehog (SHH) growth factor. If all trisomic cells show a similar reduced response to SHH, then up-regulation of the pathway in trisomic cells might ameliorate multiple DS phenotypes. We crossed Ptch1tm1Mps/+ mice, in which the canonical SHH pathway is expected to be up-regulated in every SHH-responsive cell due to the loss of function of one allele of the pathway suppressor, Ptch1, to the Ts65Dn DS model and assessed the progeny for possible rescue of multiple DS-related phenotypes. Down-regulation of Ptch produced several previously unreported effects on development by itself, complicating interpretation of some phenotypes, and a number of structural or behavioral effects of trisomy were not compensated by SHH signaling. However, a deficit in a nest-building task was partially restored in Ts;Ptch+/- mice, as were the structural anomalies of the cerebellum seen in Ts65Dn mice. These results extend the body of evidence indicating that reduced response to SHH in trisomic cells and tissues contributes to various aspects of the trisomic phenotype.

Repetitive grooming and sensorimotor abnormalities in an ephrin-A knockout model for Autism Spectrum Disorders

EphA receptors and ephrin-A ligands play important roles in neural development and synaptic plasticity in brain regions where expression persists into adulthood. Recently, EPHA3 and EPHA7 gene mutations were linked with Autism Spectrum Disorders (ASDs) and developmental neurological delays, respectively. Furthermore, deletions of ephrin-A2 or ephrin-A3, which exhibit high binding affinity for EphA3 and EphA7 receptors, are associated with subtle deficits in learning and memory behavior and abnormalities in dendritic spine morphology in the cortex and hippocampus in mice. To better characterize a potential role for these ligands in ASDs, we performed a comprehensive behavioral characterization of anxiety-like, sensorimotor, learning, and social behaviors in ephrin-A2/-A3 double knockout (DKO) mice. The predominant phenotype in DKO mice was repetitive and self-injurious grooming behaviors such as have been associated with corticostriatal circuit abnormalities in other rodent models of neuropsychiatric disorders. Consistent with ASDs specifically, DKO mice exhibited decreased preference for social interaction in the social approach assay, decreased locomotor activity in the open field, increased prepulse inhibition of acoustic startle, and a shift towards self-directed activity (e.g., grooming) in novel environments, such as marble burying. Although there were no gross deficits in cognitive assays, subtle differences in performance on fear conditioning and in the Morris water maze resembled traits observed in other rodent models of ASD. We therefore conclude that ephrin-A2/-A3 DKO mice have utility as a novel ASD model with an emphasis on sensory abnormalities and restricted, repetitive behavioral symptoms.

Delay and trace fear conditioning in C57BL/6 and DBA/2 mice: issues of measurement and performance

Strain comparison studies have been critical to the identification of novel genetic and molecular mechanisms in learning and memory. However, even within a single learning paradigm, the behavioral data for the same strain can vary greatly, making it difficult to form meaningful conclusions at both the behavioral and cellular level. In fear conditioning, there is a high level of variability across reports, especially regarding responses to the conditioned stimulus (CS). Here, we compare C57BL/6 and DBA/2 mice using delay fear conditioning, trace fear conditioning, and a nonassociative condition. Our data highlight both the significant strain differences apparent in these fear conditioning paradigms and the significant differences in conditioning type within each strain. We then compare our data to an extensive literature review of delay and trace fear conditioning in these two strains. Finally, we apply a number of commonly used baseline normalization approaches to compare how they alter the reported differences. Our findings highlight three major sources of variability in the fear conditioning literature: CS duration, number of CS presentations, and data normalization to baseline measures.

Either the dorsal hippocampus or the dorsolateral striatum is selectively involved in consolidation of forced swim-induced immobility depending on genetic background

Healthy subjects differ in the memory system they engage to learn dual-solution tasks. Both genotype and stress experience could contribute to this phenotypic variability. The present experiments tested whether the hippocampus and the dorsal striatum, the core nodes of two different memory systems, are differently involved in 24 h retention of a stress-associated memory in two genetically unrelated inbred strains of mice. Mice from both the C57BL/6J and the DBA/2J inbred strains showed progressive increase of immobility during 10 min exposure to forced swim (FS) and retrieved the acquired levels of immobility when tested 24h later. The pattern of c-fos immunostaining promoted by FS revealed activation of a large number of brain areas in both strains, including CA1 and CA3 fields of the hippocampus. However, only DBA/2J mice showed activation of the dorsolateral striatum (DLS). In addition, FS induced a positive correlation between c-fos expression in the amygdala and CA1 and CA3 in C57BL/6J mice whereas it induced a positive correlation between c-fos expression in the amygdala and DLS in DBA/2J mice. Finally, temporary post-training inactivation of the dorsal hippocampus, by local infusion of lidocaine, prevented 24h retention of immobility in C57BL/6J mice only, whereas inactivation of the DLS prevented retention in DBA/2J mice only. These findings support the view that genetic factors can determine whether the dorsal hippocampus or the DLS are selectively engaged to consolidate stress-related memory.

Prefrontal cognitive deficits in mice with altered cerebral cortical GABAergic interneurons

Alterations of inhibitory GABAergic neurons are implicated in multiple psychiatric and neurological disorders, including schizophrenia, autism and epilepsy. In particular, interneuron deficits in prefrontal areas, along with presumed decreased inhibition, have been reported in several human patients. The majority of forebrain GABAergic interneurons arise from a single subcortical source before migrating to their final regional destination. Factors that govern the interneuron populations have been identified, demonstrating that a single gene mutation may globally affect forebrain structures or a single area. In particular, mice lacking the urokinase plasminogen activator receptor (Plaur) gene have decreased GABAergic interneurons in frontal and parietal, but not caudal, cortical regions. Plaur assists in the activation of hepatocyte growth factor/scatter factor (HGF/SF), and several of the interneuron deficits are correlated with decreased levels of HGF/SF. In some cortical regions, the interneuron deficit can be remediated by endogenous overexpression of HGF/SF. In this study, we demonstrate decreased parvalbumin-expressing interneurons in the medial frontal cortex, but not in the hippocampus or basal lateral amygdala in the Plaur null mouse. The Plaur null mouse demonstrates impaired medial frontal cortical function in extinction of cued fear conditioning and the inability to form attentional sets. Endogenous HGF/SF overexpression increased the number of PV-expressing cells in medial frontal cortical areas to levels greater than found in wildtype mice, but did not remediate the behavioral deficits. These data suggest that proper medial frontal cortical function is dependent upon optimum levels of inhibition and that a deficit or excess of interneuron numbers impairs normal cognition.

Sex-specific positive and negative consequences of avoidance training during childhood on adult active avoidance learning in mice

In humans and animals cognitive training during childhood plays an important role in shaping neural circuits and thereby determines learning capacity later in life. Using a negative feedback learning paradigm, the two-way active avoidance (TWA) learning, we aimed to investigate in mice (i) the age-dependency of TWA learning, (ii) the consequences of pretraining in childhood on adult learning capacity and (iii) the impact of sex on the learning paradigm in mice. Taken together, we show here for the first time that the beneficial or detrimental outcome of pretraining in childhood depends on the age during which TWA training is encountered, indicating that different, age-dependent long-term "memory traces" might be formed, which are recruited during adult TWA training and thereby either facilitate or impair adult TWA learning. While pretraining during infancy results in learning impairment in adulthood, pretraining in late adolescence improved avoidance learning. The experiments revealed a clear sex difference in the group of late-adolescent mice: female mice showed better avoidance learning during late adolescence compared to males, and the beneficial impact of late-adolescent pretraining on adult learning was more pronounced in females compared to males.

Factors affecting graded and ungraded memory loss following hippocampal lesions

This review evaluates three current theories--Standard Consolidation (Squire & Wixted, 2011), Overshadowing (Sutherland, Sparks, & Lehmann, 2010), and Multiple Trace-Transformation (Winocur, Moscovitch, & Bontempi, 2010)--in terms of their ability to account for the role of the hippocampus in recent and remote memory in animals. Evidence, based on consistent findings from tests of spatial memory and memory for acquired food preferences, favours the transformation account, but this conclusion is undermined by inconsistent results from studies that measured contextual fear memory, probably the most commonly used test of hippocampal involvement in anterograde and retrograde memory. Resolution of this issue may depend on exercising greater control over critical factors (e.g., contextual environment, amount of pre-exposure to the conditioning chamber, the number and distribution of foot-shocks) that can affect the representation of the memory shortly after learning and over the long-term. Research strategies aimed at characterizing the neural basis of long-term consolidation/transformation, as well as other outstanding issues are discussed.

Aging in the cerebellum and hippocampus and associated behaviors over the adult life span of CB6F1 mice

In the present study we examined the effects of normal aging in the hippocampus and cerebellum, as well as behaviors associated with these substrates. A total of 67 CB6F1 hybrid mice were tested at one of five ages (4, 8, 12, 18 or 25 months) on the context pre-exposure facilitation effect (CPFE) modification of fear conditioning, rotorod, Barnes maze, acoustic startle, Morris water maze (MWM) and 500-ms trace eyeblink classical conditioning (EBCC). Behavioral tasks were chosen to increase the ability to detect age-related changes in learning, as trace EBCC is considered a more difficult paradigm (compared to delay EBCC) and the CPFE has been found to be more sensitive to hippocampus insults than standard contextual fear conditioning. To assess the effects of age on the brain, hippocampus volume was calculated and unbiased stereology was used to estimate the number of Purkinje neurons in the cerebellar cortex. A significant, age-related loss of Purkinje neurons was found-beginning at 12 months of age-and hippocampus volume remained stable over the adult life span. Age-related impairment was found, beginning at 12-18 months in the rotorod, and mice with fewer Purkinje neurons showed greater impairment in this task. CB6F1 mice retained auditory acuity across the life span and mice aged 25 months showed significant age-related impairment in the EBCC task; however, deficits were not associated with the loss of Purkinje neurons. Although the CPFE task is considered more sensitive to hippocampus insult, no age-related impairment was found. Spatial memory retention was impaired in the Barnes maze at 25 months, but no significant deficits were seen in the MWM. These results support the finding of differential aging in the hippocampus and cerebellum.

Pre-attentive, context-specific representation of fear memory in the auditory cortex of rat

Neural representation in the auditory cortex is rapidly modulated by both top-down attention and bottom-up stimulus properties, in order to improve perception in a given context. Learning-induced, pre-attentive, map plasticity has been also studied in the anesthetized cortex; however, little attention has been paid to rapid, context-dependent modulation. We hypothesize that context-specific learning leads to pre-attentively modulated, multiplex representation in the auditory cortex. Here, we investigate map plasticity in the auditory cortices of anesthetized rats conditioned in a context-dependent manner, such that a conditioned stimulus (CS) of a 20-kHz tone and an unconditioned stimulus (US) of a mild electrical shock were associated only under a noisy auditory context, but not in silence. After the conditioning, although no distinct plasticity was found in the tonotopic map, tone-evoked responses were more noise-resistive than pre-conditioning. Yet, the conditioned group showed a reduced spread of activation to each tone with noise, but not with silence, associated with a sharpening of frequency tuning. The encoding accuracy index of neurons showed that conditioning deteriorated the accuracy of tone-frequency representations in noisy condition at off-CS regions, but not at CS regions, suggesting that arbitrary tones around the frequency of the CS were more likely perceived as the CS in a specific context, where CS was associated with US. These results together demonstrate that learning-induced plasticity in the auditory cortex occurs in a context-dependent manner.

Role of mGluR4 in acquisition of fear learning and memory

Group III metabotropic glutamate receptors (mGluRs), which are generally located presynaptically, modulate synaptic transmission by regulating neurotransmitter release. Previously we showed enhanced amygdala-dependent cued fear conditioning in mGluR4(-/-) mice 24 h following training involving two tone-shock pairings. In this study, we assessed the effects of modulating mGluR4 signaling on acquisition and extinction of conditioned fear. mGluR4(-/-) and wild-type female and male mice received 10 tone-shock pairings during training. Compared to wild-type mice, mGluR4(-/-) mice showed enhanced acquisition and extinction of cued fear. Next, we assessed whether acute pharmacological stimulation of mGluR4 with the specific orthosteric mGluR4 agonist LSP1-2111 also affects acquisition and extinction of cued fear. Consistent with the enhanced acquisition of cued fear in mGluR4(-/-), LSP1-2111, at 2.5 and 5 mg/kg, inhibited acquisition of cued fear conditioning in wild-type male mice. The drug's effect on extinction was less clear and only a subtle effect was seen at 5 mg/kg. Finally, analysis of microarray data of amygdala tissues from mGluR4(-/-) versus wild-type and from wild-type mice treated with a mGluR4 agonist versus saline revealed a significant overlap in pattern of gene expression. Together, these data support a role for mGluR4 signaling in acquisition of fear learning and memory. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Can zebrafish learn spatial tasks? An empirical analysis of place and single CS-US associative learning

The zebrafish may be an ideal tool with which genes underlying learning and memory can be identified and functionally investigated. From a translational viewpoint, relational learning and episodic memory are particularly important as their impairment is the hallmark of prevalent human neurodegenerative diseases. Recent reports suggest that zebrafish are capable of solving complex relational-type associative learning tasks, namely spatial learning tasks. However, it is not known whether good performance in these tasks was truly based upon relational learning or upon a single CS-US association. Here we study whether zebrafish can find a rewarding stimulus (sight of conspecifics) based upon a single associative cue or/and upon the location of the reward using a method conceptually similar to 'context and cue dependent fear conditioning' employed with rodents. Our results confirm that zebrafish can form an association between a salient visual cue and the rewarding stimulus and at the same time they can also learn where the reward is presented. Although our results do not prove that zebrafish form a dynamic spatial map of their surroundings and use this map to locate their reward, they do show that these fish perform similarly to rodents whose hippocampal function is unimpaired. These results further strengthen the notion that complex cognitive abilities exist in the zebrafish and thus they may be analyzed using the excellent genetic tool set developed for this simple vertebrate.

A parametric analysis of factors affecting acquisition and extinction of contextual fear in C57BL/6 and DBA/2 mice

Behavioral analyses of genetically modified and inbred strains of mice have revealed neural systems and molecules that are involved in memory formation. Many of these studies have examined memories that form in contextual fear conditioning, in which an organism learns that a particular context signals the occurrence of a footshock. During fear extinction, nonreinforced exposure to the context results in the loss of the conditioned fear response. The study of extinction has been instrumental for behavioral and molecular theories of memory. However, many of the transgenic, knockout, and inbred strains of mice that have been widely studied in memory have behavioral deficits in contextual fear conditioning, which makes the study of extinction in these mice particularly challenging. Here we explore several strategies for studying extinction in C57BL/6 and DBA/2 mice, two strains known to differ in contextual fear conditioning. First, we attempt to equate performance prior to extinction through several extensive conditioning protocols. Second, we examine extinction in subsets of mice matched for initial levels of context conditioning. Third, we examine within-strain effects of variables known to affect extinction. Differences between the strains persisted across extensive conditioning and extinction protocols, but both strains were sensitive to session duration and context manipulations during extinction. We describe the implications of our results for behavioral and neurobiological approaches to extinction, and we examine the general challenges in studying extinction in subjects that differ in learning or performance prior to extinction.

Comparison of the performance of DBA/2 and C57BL/6 mice in transitive inference and foreground and background contextual fear conditioning

DBA/2 mice have altered hippocampal structure and perform poorly in several hippocampus-dependent contextual/spatial learning tasks. The performance of this strain in higher cognitive tasks is less studied. Transitive inference is a hippocampus-dependent task that requires an abstraction to be made from prior rules to form a new decision matrix; performance of DBA/2 mice in this task is unknown, whereas contextual fear conditioning is a hippocampus-dependent task in which DBA/2 mice have deficits. The present study compared DBA/2J and C57BL/6J inbred mice in two different contextual fear conditioning paradigms and transitive inference to test whether similar deficits are seen across these hippocampus-dependent tasks. For background fear conditioning, mice were trained with two paired presentations of an auditory conditioned stimulus (CS, 30 seconds, 85 dB white noise) paired with an unconditioned stimulus (US, 2 seconds, 0.57 mA footshock), the context was a continuous background CS. Mice were tested for contextual learning 24 hours later. Foreground fear conditioning differed in that no auditory CS was presented. For transitive inference, separate mice were trained to acquire a series of overlapping odor discrimination problems and tested with novel odor pairings that either did or did not require the use of transitive inference. DBA/2 mice performed significantly worse than the C57BL/6 in both foreground and background fear conditioning and transitive inference. These results demonstrate that the DBA/2 mice have deficits in higher-cognitive processes and suggest that similar substrates may underlie deficits in contextual learning and transitive inference.

Animated bird silhouette above the tank: acute alcohol diminishes fear responses in zebrafish

Alcohol dependence and alcohol abuse represent major unmet medical needs. The zebrafish is considered to be a promising vertebrate species with which the effects of alcohol on brain function and behavior and the mechanisms underlying these effects may be studied. Alcohol is known to induce alterations in motor function as well as fear and anxiety. Here we utilize a recently developed fear paradigm in which we employ an animated (moving) image of a bird silhouette. We measure the effect of acute alcohol administration (dose range employed: 0.00-0.75 vol/vol percentage, bath exposure for 60 min) on the behavioral responses of zebrafish. We test these responses during a pre-stimulus, stimulus and post-stimulus period of the task using both a video-tracking and an observation based quantification method. The fear inducing stimulus was found to decrease the distance of the zebrafish from the bottom of the tank, to increase number of erratic movements, and to increase the number of jumps in alcohol exposed fish (versus control fish). Alcohol attenuated these fear responses in a dose dependent manner. In addition, alcohol decreased general activity at the highest dose, an effect that was independent of the presentation of the stimulus. We discuss the similarities and differences between observation and video-tracking based results and conclude that fear paradigms will be useful in revealing alcohol induced functional changes in the brain of zebrafish.

Fear relief-toward a new conceptual frame work and what endocannabinoids gotta do with it

The endocannabinoid system seems to play very specific roles in fear extinction, which can only be described within a well-defined model of the various fear relief processes. We, therefore, seek to clarify the current conceptual framework of fear relief within classical and operant fear conditioning paradigms as well as propose new clarifications within this framework where necessary. Based on these revisions as well as previous research involving the endocannabinoid system and fear relief, we are able to pinpoint the processes in which endocannabinoids seem to play a significant role. Following auditory-cued fear conditioning, this applies in particular to habituation and its involvement in acute and long-lasting fear relief. Following contextual conditioning, in contrast, endocannabinoids seem to affect relearning processes as well. Furthermore, we describe how the involvement of the endocannabinoid system develops over the course of the fear relief process and what this may imply for the clinical use of pharmacotherapies targeting the endocannabinoid system in treating fear and anxiety disorders.

Parametric and pharmacological modulations of latent inhibition in mouse inbred strains

Latent inhibition (LI) is a cross species selective attention phenomenon, which is disrupted by amphetamine and enhanced by antipsychotic drugs (APDs). Accumulating data of LI in gene-modified mice as well as in mouse inbred strains suggest genetic component of LI. Here we study modulation of LI in mouse inbred strains with spontaneously disrupted LI by parametric manipulations (number of pre-exposures and conditioning trials) and pharmacological treatments with antipsychotics and NMDA modulator, D-serine. C3H/He and CBA/J inbred mice showed disrupted LI under conditions with 40 pre-exposures (PE) and 2 trials of the conditioned stimulus-unconditioned stimulus (CS-US) due to either loss of the pre-exposure effect or a ceiling effect of poor learning, respectively. The increased number of pre-exposures and/or number of conditioning trials corrected expression of LI in these inbred mice. The disrupted LI was also reversed by haloperidol in both inbred strains at 1.2 mg/kg but not at 0.4 mg/kg, as well as by clozapine (at 3 mg/kg in C3H/He and at 9 mg/kg in CBA/J mice). D-serine potentiated LI in C3H/He mice at 600 mg/kg, but not in the CBA/J at both studied doses (600 and 1800 mg/kg). Desipramine (10 mg/kg) had no effect on LI in both inbred mouse strains. Our findings demonstrated some resemblance between the effects of parametric and pharmacological manipulations on LI, suggesting that APDs may affect the capacity of the brain processes environmental stimuli in LI. Taken together, LI may offer a translational strategy that allows prediction of drug efficacy for cognitive impairments in schizophrenia.

Age sensitivity of behavioral tests and brain substrates of normal aging in mice

Knowledge of age sensitivity, the capacity of a behavioral test to reliably detect age-related changes, has utility in the design of experiments to elucidate processes of normal aging. We review the application of these tests in studies of normal aging and compare and contrast the age sensitivity of the Barnes maze, eyeblink classical conditioning, fear conditioning, Morris water maze, and rotorod. These tests have all been implemented to assess normal age-related changes in learning and memory in rodents, which generalize in many cases to age-related changes in learning and memory in all mammals, including humans. Behavioral assessments are a valuable means to measure functional outcomes of neuroscientific studies of aging. Highlighted in this review are the attributes and limitations of these measures in mice in the context of age sensitivity and processes of brain aging. Attributes of these tests include reliability and validity as assessments of learning and memory, well-defined neural substrates, and sensitivity to neural and pharmacological manipulations and disruptions. These tests engage the hippocampus and/or the cerebellum, two structures centrally involved in learning and memory that undergo functional and anatomical changes in normal aging. A test that is less well represented in studies of normal aging, the context pre-exposure facilitation effect (CPFE) in fear conditioning, is described as a method to increase sensitivity of contextual fear conditioning to changes in the hippocampus. Recommendations for increasing the age sensitivity of all measures of normal aging in mice are included, as well as a discussion of the potential of the under-studied CPFE to advance understanding of subtle hippocampus-mediated phenomena.

Systemic stimulation of TLR2 impairs neonatal mouse brain development

BACKGROUND

Inflammation is associated with perinatal brain injury but the underlying mechanisms are not completely characterized. Stimulation of Toll-like receptors (TLRs) through specific agonists induces inflammatory responses that trigger both innate and adaptive immune responses. The impact of engagement of TLR2 signaling pathways on the neonatal brain is still unclear. The aim of this study was to investigate the potential effect of a TLR2 agonist on neonatal brain development.

METHODOLOGY/PRINCIPAL FINDINGS

Mice were injected intraperitoneally (i.p.) once a day from postnatal day (PND) 3 to PND11 with endotoxin-free saline, a TLR2 agonist Pam(3)CSK(4) (5 mg/kg) or Lipopolysaccharide (LPS, 0.3 mg/kg). Pups were sacrificed at PND12 or PND53 and brain, spleen and liver were collected and weighed. Brain sections were stained for brain injury markers. Long-term effects on memory function were assessed using the Trace Fear Conditioning test at PND50. After 9 days of Pam(3)CSK(4) administration, we found a decreased volume of cerebral gray matter, white matter in the forebrain and cerebellar molecular layer that was accompanied by an increase in spleen and liver weight at PND12. Such effects were not observed in Pam3CSK4-treated TLR 2-deficient mice. Pam3CSK4-treated mice also displayed decreased hippocampus neuronal density, and increased cerebral microglia density, while there was no effect on caspase-3 or general cell proliferation at PND12. Significantly elevated levels of IL-1β, IL-6, KC, and MCP-1 were detected after the first Pam3CSK4 injection in brain homogenates of PND3 mice. Pam(3)CSK(4) administration did not affect long-term memory function nor the volume of gray or white matter.

CONCLUSIONS/SIGNIFICANCE

Repeated systemic exposure to the TLR2 agonist Pam(3)CSK(4) can have a short-term negative impact on the neonatal mouse brain.

Associative learning in zebrafish (Danio rerio)

The zebrafish has been one of the primary study species utilized in developmental biology. However, it is also gaining increasing amount of interest in other disciplines of biology including behavioral neuroscience; the numerous genetic tools developed and the large amount of genetic information accumulated for this species by now make it an excellent tool for the analysis of the mechanisms of complex central nervous system characteristics. Although several studies have investigated the biological and genetic underpinnings of associative learning (and memory), given the complexity of these phenomena, much remains to be discovered. In the past, the zebrafish has been employed particularly successfully in screening applications where a large number of mutations or drug effects had to be analyzed. Briefly, the practical simplicity and system complexity of the zebrafish may make this species an excellent tool also for the analysis of the mechanisms of associative learning. Screening, however, requires appropriate phenotypical (in this case behavioral) paradigms. A step in this direction is the characterization of learning abilities of zebrafish. The number of studies focused on cognitive and/or mnemonic characteristics of zebrafish is orders of magnitude smaller than those with rats or mice, but recently zebrafish has also started to be utilized in this research. The current chapter reviews these most recent developments. It also discusses certain unique features of zebrafish that must be taken into account when designing an associative learning task and how these tasks may be made high throughput.

Altered synaptic plasticity and behavioral abnormalities in CNGA3-deficient mice

The role of the cyclic nucleotide-gated (CNG) channel CNGA3 is well established in cone photoreceptors and guanylyl cyclase-D-expressing olfactory neurons. To assess a potential function of CNGA3 in the mouse amygdala and hippocampus, we examined synaptic plasticity and performed a comparative analysis of spatial learning, fear conditioning and step-down avoidance in wild-type mice and CNGA3 null mutants (CNGA3(-/-) ). CNGA3(-/-) mice showed normal basal synaptic transmission in the amygdala and the hippocampus. However, cornu Ammonis (CA1) hippocampal long-term potentiation (LTP) induced by a strong tetanus was significantly enhanced in CNGA3(-/-) mice as compared with their wild-type littermates. Unlike in the hippocampus, LTP was not significantly altered in the amygdala of CNGA3(-/-) mice. Enhanced hippocampal LTP did not coincide with changes in hippocampus-dependent learning, as both wild-type and mutant mice showed a similar performance in water maze tasks and contextual fear conditioning, except for a trend toward higher step-down latencies in a passive avoidance task. In contrast, CNGA3(-/-) mice showed markedly reduced freezing to the conditioned tone in the amygdala-dependent cued fear conditioning task. In conclusion, our study adds a new entry on the list of physiological functions of the CNGA3 channel. Despite the dissociation between physiological and behavioral parameters, our data describe a so far unrecognized role of CNGA3 in modulation of hippocampal plasticity and amygdala-dependent fear memory.

Strain difference in the effect of infralimbic cortex lesions on fear extinction in rats

The infralimbic division of the medial prefrontal cortex (IL) has been implicated in the consolidation and retention of extinction memories. However, the effects of IL lesions on the retention of extinction memory are inconsistent. In the present experiments, we examined whether rat strain influences the effects of IL lesions on extinction. In Experiment 1, Sprague-Dawley (SD) or Long-Evans (LE) rats received a standard auditory fear conditioning procedure, which was followed by an extinction session; freezing served as the index of conditional fear. Our results reveal that focal IL lesions impair the retention of extinction in SD, but not LE rats. In addition to the strain difference in sensitivity to IL lesions, LE rats exhibited significantly higher levels of contextual fear before the outset of extinction training than SD rats. In a second experiment we thus examined whether contextual fear influenced the sensitivity of extinction to IL lesions in LE rats. LE rats received the same conditioning as in Experiment 1, and then were either merely exposed to a novel context or administered unsignaled shocks in that context, followed by extinction and test sessions. Our results reveal that LE rats with IL lesions showed normal extinction regardless of the levels of contextual fear manifest before extinction. Thus, we conclude that rat strain is an important variable that influences the role of infralimbic cortex in fear extinction.